This invention relates to an Al-Mg type alloy characterized by an extremely fine grain size and outstanding strength in the fully recrystallized or dead soft condition.
The Al-Mg alloy system is well known to provide excellent strength, corrosion resistance and fracture toughness. In the fully annealed or dead soft condition, the yield strengths of the alloys of this system increase with increased Mg content, reaching about 138 MPa (20 ksi) in alloys containing about 5% Mg. Further magnesium additions provide slightly higher yield strengths, but such alloys are difficult to process and often exhibit a pronounced susceptibility to stress corrosion cracking. As a result, in the past when yield strengths above 138 MPa (20 ksi) are desired, it has been necessary to produce the Al-Mg alloy materials in a partially annealed condition, at a significant sacrifice in ductility and thermal stability.
The post-casting processing procedures for the commercially produced Al-Mg alloys are closely controlled to obtain a small uniform grain structure in the final material. However, under the best of commercial fabricating practices, the average grain size for the Al-Mg type alloy products will usually be ASTM No. 8 or larger as determined by ASTM Standard No. E112-63, which is equivalent to an average grain diameter of about 20 microns. Even under laboratory conditions, an average grain size of less than ASTM No. 9, indicating an average grain diameter smaller than 16 microns, has heretofore been very difficult to obtain for most aluminum alloys.
It is well recognized that a finer grain size generally provides a higher yield strength in a fully annealed product and, moreover, an ultrafine grain size may also indicate that the materials has some degree of superplasticity.
It is against this background that the present invention was developed.